1. Field of the Invention
The present invention relates to a method for setting boundary values for error diffusion, and to an image signal processing method and apparatus using the boundary values set by the method. The present invention further relates to a printing apparatus employing the image processing technique.
2. Description of the Related Art
One conversion technique for multi-tone image data into data suitable for binary representation indicating the on and off states of each pixel is an error diffusion method. In the error diffusion method, density error caused in binarization is stored and used for processing neighboring pixels. The error diffusion method allows density information to be stored even after binarization. For example, approximately three to eight tonal levels per pixel can be stored.
Generally, in error-diffusing multi-tone image data, tones of the original image data are equally divided into multiple parts, and a boundary value for error diffusion is then determined. In this approach, the visual change from a non-printing state (level 0) to the first level (level 1) is larger than the visual change from level 1 to the second level (level 2).
If the visual change of tones between level 0 and level 1 and the visual change of tones between level 1 and level 2 are equivalently handled, the former visual change becomes greater than the latter visual change.
Likewise, the visual change of tones between level 1 and level 2 becomes greater than the visual change of tones between level 2 and level 3, and the visual change of tones between level 2 and level 3 becomes greater than the visual change of tones between level 3 and level 4.
For example, in a 256-grayscale (8-bit) image, stepped tones in a highlighted portion are more noticeable. One solution to this problem is disclosed in Floyd, R. and Steinberg, L., “An Adaptive Algorithm for Spatial Gray Scale,” SID DIGEST, 1975. In this solution, the number of tones used for image processing is increased from 8 bits (256 tones) to 10 bits (1024 tones) or 12 bits (4096 tones) to reduce visual change of tones.
However, this method causes a high-density portion without substantial tonal difference to be finely divided more than necessary. Thus, a large memory capacity is required.